By the method of fabricating improved back reflective substrates disclosed herein, photoresponsive devices may be manufactured. Such devices (1) exhibit increased operational reliability, (2) exhibit increased operational efficiency, (3) are fabricated in a manner resulting in increased yields, and (4) have a markedly reduced number of surface defects, thereby substantially reducing the number of (a) current-shunting paths and (b) nucleation centers which promote the nonhomogeneous growth of the subsequently deposited semiconductor material. The present invention has particular applicability to (1) large area, amorphous photovoltaic devices wherein the active semiconductor elements thereof are deposited onto a substrate electrode as relatively thin layers and are subsequently covered by a second electrode, and (2) the fabrication of such thin film, large area photovoltaic devices from amorphous semiconductor alloys. In accordance with the principles of the present invention, the method of fabrication is adapted to fabricate said substrate electrode with a greatly reduced number of surface defects so as to present a morphologically smooth and level substrate surface upon which homogeneous semiconductor material may be grown.
Single crystal photovoltaic devices, especially crystalline silicon photovoltaic devices have been utilized for some time as sources of electrical power because they are inherently non-polluting, silent and consume no expendable natural resources in their operation. However, the utility of such devices is limited by problems associated with the manufacture thereof. More particularly, single crystal materials (1) are difficult to produce in sizes substantially larger than several inches in diameter, (2) are thicker and heavier than their amorphous counterparts; and (3) are expensive and time consuming to fabricate.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor materials, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n type photovoltaic devices which are, in operation, substantially equivalent to their crystalline counterparts. It is to be noted that the term "amorphous", as used herein, includes all materials or alloys which have no long range order, although they may have short or intermediate range order or even contain, at times, crystalline inclusions.
It is now possible to prepare amorphous silicon alloys by glow discharge or vacuum deposition techniques, said alloys possessing (1) acceptable concentrations of localized defect states in the energy gaps thereof, and (2) high quality electrical and optical properties. Such deposition techniques are fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors, issued to Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; U.S. Pat. No. 4,217,374, of Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, also entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors; and U.S. patent application Ser. No. 423,424 of Stanford R. Ovshinsky, David D. Allred, Lee Walter, and Stephen J. Hudgens entitled Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy. As disclosed in these patents and application, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized defect states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell a large band gap material absorbs only the short wavelength light, while in subsequent cells smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant. The multiple cells preferably include a back reflector for increasing the percentage of incident light reflected from the substrate back through the semiconductor layers of the cells. It should be obvious that the use of a back reflector, by increasing the use of light entering the cell, increases the operational efficiency of the multiple cells. Accordingly, it is important that any layer deposited atop the light incident surface of the substrate be transparent so as to pass a high percentage of incident light from the reflective surface of the back reflector through the semiconductor layers.
Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in U.S. Pat. No. 4,400,409, issued Aug. 23,1983, for A Method Of Making P-Doped Silicon Films And Devices Made Therefrom; and pending U.S. patent applications: Ser. No. 244,386, filed Mar. 16, 1981, for Continuous Systems For Depositing Amorphous Semiconductor Material; Ser. No. 240,493, filed Mar. 16, 1981, for Continuous Amorphous Solar Cell Production System; Ser. No. 306,146, filed Sept. 28, 1981, for Multiple Chamber Deposition And Isolation System And Method; Ser. No. 359,825, filed Mar. 19, 1982, for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells; and Ser. No. 460,629 filed Jan. 24, 1983 for Method and Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a photovoltaic device of p-i-n type configurations, the first chamber is dedicated for depositing a p-type semiconductor alloy, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for depositing an n-type semiconductor alloy. Since each deposited semiconductor alloy, and especially the intrinsic semiconductor alloy, must be of high purity; every possible precaution is taken to insure that the sanctity of the vacuum envelope formed by the various chambers of the deposition apparatus remains uncontaminated by impurities, regardless of origin.
The layers of semiconductor alloy material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form photoresponsive devices, such as, but not limited to photovoltaic cells which include one or more p-i-n cells or one or more n-i-p cells, Schottky barriers, photodiodes, phototransistors, or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained.
As is obvious from the foregoing, thin film amorphous semiconductor materials offer several distinct advantages over crystalline materials, insofar as they can be easily and economically fabricated by newly developed mass production processes. However, in the fabrication of thin film semiconductor material by the aforementioned deposition processes, the presence of defects which cause low resistance current paths and nonhomogeneous growth of semiconductor material have been noted. The low resistance current paths and the nonhomogeneous growth have (1) seriously impaired the performance of the photoresponsive devices fabricated from the semiconductor material, and (2) detrimentally affected the production yields of that semiconductor material. The defects which give rise to low resistance current paths and nonhomogeneous growth were previously thought to either (1) be present in the morphology of the substrate electrode, or (2) develop during the deposition of the semiconductor layers. Commonly assigned U.S. patent application Ser. No. 518,184 filed May 28, 1983, entitled Barrier Layer For Photovoltaic Devices of P. Nath and M. Izu relates to one method of improving the performance of photoresponsive devices despite the presence of those defects which initiate current-shunting paths through the semiconductor material. Further, copending U.S. patent application Ser. No. 586,635, filed Mar. 5, 1984 of Prem Nath, Masatsugu Izu, Herbert Ovshinsky, Clifford Tennenhouse and James Young, and assigned to the assignee of the instant invention, defines the current shunting paths and the nonhomogeneous growth of semiconductor material to be due to the tens of thousands of surface defects which are present on the deposition surface of even high grade stainless steel. The solution proposed in said patent application was to electroplate a relatively thick (5-12 micron) nickel alloy levelling layer atop the deposition surface of the substrate so as to provide a substantially defect-free surface upon which to deposit the semiconductor material. However, while the aforementioned levelling layer effectively reduced the number of those substrate surface defects which give rise to current shunts and nonhomogeneous growth centers, the process is expensive, requires the deposition of a very thick layer atop the stainless steel or aluminum substrate, and still requires the deposition of a specular or diffuse reflective veneer thereupon. In contrast thereto, the instant invention relates to the fabrication of back reflective substrates for photosensitve devices, said substrates not only having a deposition surface characterized by a substantially reduced number of surface defects, but also by a reduced number of production steps and by a more economical process than heretofore possible.
Before proceeding with a description of the present invention, it is necessary to describe in greater detail the problems which the present invention is capable of solving. Accordingly, the paragraphs which follow first explain the difficulties caused by defects formed in the deposition surface of the substrate, and then detail the manner in which they have been solved. Dealing first with the problem of current-shunting defects, (and thereafter with the problem of nucleation defects), said defects may be characterized as "shunts", or "short-circuit" defects. In order to understand the operation thereof, it is important to note the thicknesses of the deposited semiconductor layers in a photoresponsive device such as a photovoltaic cell (also referred to as a solar cell). A typical p layer may be only on the order of 400 angstroms thick, a typical i layer may be only on the order of 3500 angstroms thick, and a typical n layer may be only on the order of 200 angstroms thick, thereby providing a total thickness of the semiconductor material for a single p-i-n photovoltaic cell of only about 4100 angstroms. It should therefore be appreciated that substrate defects, i.e., surface irregularities, however small, may not be readily covered by the thin film of deposited semiconductor material.
Current-shunting defects are present when one or more low resistance current paths develop between the electrodes of, for example, the solar cell. Under operating conditions, a solar cell in which a shunt has developed, exhibits either (1) a low power output, since electrical current collected at the electrodes thereof flows through the defect region (the path of least resistance) in preference to an external load, or (2) complete failure, where sufficient current is shunted through the defect region to "burn out" the cell.
Current-shunting defects or defect regions, the terms being interchangeably used herein, are not limited to "overt" or "patent" low resistance current paths. In some cases the adverse effects of a defect are latent and not immediately manifested. Latent defects can give rise to what will be referred to hereinafter as an "operational mode failure", wherein a photovoltaic device, initially exhibiting satisfactory electrical performance, suddenly fails. The failures will be referred to in this application as operational mode failures regardless of whether or not the device was previously connected to a load for the generation of power, it only being necessary that the device was, at some time, subjected to illumination, thereby initiating the generation of charge carriers. It has now been ascertained that the current-shunting defects, both latent and patent, arise from surface defects or irregularities in the morphology of the deposition surface of the substrate material, said defects being of a type which are either (1) not uniformly covered by or (2) cause nonhomogeneous growth of the subsequently deposited thin film layers of semiconductor material.
Despite the use of the highest quality stainless steel to serve as the substrate or base electrode upon which the semiconductor material is deposited, it has been estimated that from 10,000 to 100,000 surface defects (i.e., irregularities) per square centimeter are present on the deposition surface thereof. Such irregularities take the form of projections, craters, or other deviations from a smooth finish, and may be under a micron in (1) depth below the surface, (2) height above the surface, or (3) diameter. Depending upon their configuration, size, the sharpness with which the irregularities deviate from a smooth surface finish, and the manner in which the semiconductor material covers or fails to cover the defects, a low resistance current path through the semiconductor material may be established, thereby effectively short-circuiting the two electrodes of, for instance, the photovoltaic cell which has been fabricated therefrom. This may occur in numerous ways. For instance, a spike projecting from the surface of the substrate electrode may be of too great a height to be covered by the subsequent deposition of semiconductor material, and therefore, be in direct electrical contact with the second electrode when that second electrode is deposited atop the semiconductor material. Likewise, a crater formed in the surface of the substrate electrode may be of too large a diameter or too large a depth to be filled by the subsequent deposition of semiconductor material and therefore, be in sufficient proximity to the second electrode, when that second electrode is deposited atop the semiconductor material, for electrical current to either (1) bridge the gap which exists between the two electrodes, or (2) through actual use (the photo-induced generation of electrical current) of the photovoltaic device, cause the material of one of the electrodes to migrate toward and contact the other of the electrodes, and thereby pass electrical current therebetween. Note that even if the size of the defect (measured as its deviation from a smooth surface) is not very large, but it includes one or more sharp or jagged features, said defect is still capable of causing the deposited semiconductor material to be of less than optimum quality. This is because the sharp features of even small defects are (1) very difficult to uniformly cover by the subsequently deposited semiconductor material, and (2) capable of forming nucleation centers which promote nonhomogeneous growth of the subsequently deposited semiconductor material. Therefore, it is a first object of the instant invention to eliminate both defects which (1) due to the size thereof, cannot be adequately covered by the subsequently deposited semiconductor material, and defects which (2) due to the sharp features thereof, inhibit the deposition of homogeneous layers of semiconductor material.
The instant invention, as will be described in greater detail hereinbelow, provides for the fabrication of improved substrates specifically adapted for use in the fabrication of photoresponsive devices, said substrates being lightweight, relatively thin, easily and economically manufactured, and adapted to substantially eliminate the cause of low resistance current paths, both patent and latent, as well as to eliminate nonhomogeneous semiconductor growth caused by nucleation centers. More particularly, semiconductor devices produced in accordance with the principles outlined by the subject disclosure are characterized by improved production yields, improved electrical output and hence higher efficiencies, and a substantial reduction of substrate surface irregularities, said irregularities normally giving rise to current-shunting defects and/or nucleation centers.
According to a further feature of the preferred embodiment of the instant invention, the thin electroplated metallic substrate, which includes a substantially defect-free deposition surface upon which to subsequently deposit amorphous semiconductor alloy layers, may also be specifically tailored to provide back reflection of incident light.
The back reflective function can be inexpensively designed into the process for electroplating the substrate of the present invention without increasing the number of surface defects thereon. More particularly, in the manufacture of photoresponsive devices, the efficiency of the devices may be increased by forming back reflectors on the surface of the substrate upon which the amorphous semiconductor materials are subsequently deposited. These back reflectors may be either specular or diffuse. With either type of reflector, light which has initially passed through the active semiconductor region of the devices, but which is unabsorbed or unused on its initial pass, is redirected from the back surface of the device by the reflector to pass, once again, through those active semiconductor regions. The additional pass results in increased photon absorption and charge carrier generation in the active regions, thereby providing increased short circuit currents. In the case of specular back reflectors, the unused light is generally redirected for one or more additional passes through the active semiconductor regions of the device. And, in the case of diffuse back reflectors, the light is scattered in addition to being redirected through the active regions, thereby mandating that a portion of the redirected light travels at angles sufficient to cause it to be substantially confined within the device by internal reflection. In this manner, the devices sense and utilize the multiple reflections of the redirected light through the active semiconductor regions. As a result, both specular and diffuse back reflectors provide for increased short circuit currents and thus increased efficiencies of the photoresponsive devices with which they are employed. Another advantage of the diffuse reflector is that since the directed light passes through the active regions of the device at an angle, the active semiconductor regions can be made thinner than otherwise possible to reduce charge carrier recombination, while still maintaining efficient charge carrier generation and collection.
Heretofore, problems existed or great difficulty was experienced with the methods employed, in forming either specular or diffuse back reflectors on the deposition surfaces of substrates. In the case of specular reflectors, it was (prior to the instant invention) difficult to deposit a mirror-like layer of a reflective material, such as gold, silver, aluminum, copper, chromium or molybdenum, onto the substrate in a manner which would not subsequently peel. In the case of diffuse reflectors, a roughened texture was achieved by techniques such as sandblasting the substrate surface. However, such texturizing techniques resulted in the formation of morphological projections, craters and other uncontrollable defects. As previously stressed, due to the thinness of the deposited body of semiconductor material, the presence of defects of controlled size, gave rise to low resistance current paths and nucleation centers which rendered the photoresponsive device useless. The problems heretofore encountered in the production of diffuse and specular back reflectors are solved by the use of the electroplated substrate of the present application. Simply stated, it is now possible to electroplate a substrate characterized by a morphologically smooth, substantially defect-free, diffuse and/or specular back reflective surface upon which semiconductor alloy material can be deposited without developing low resistance current paths or centers of nucleation for the nonhomogeneous growth of semiconductor material.
Finally, the ultra-thin, substantially defect-free substrate of the instant invention offers significant advantages in the fabrication of electrically isolated, but electrically-interconnected small area segments from a large area photoresponsive device such as a photovoltaic cell. In order to better understand the advantages afforded by the substrate of the instant invention, the importance of dividing the large area cell into small area segments will first be explained.
As discussed hereinabove and detailed hereinafter, the effects of shorts and shunts may be mitigated or intensified, depending upon the current collection design of the large area device. More particularly, if a large area semiconductor body was designated to form a single, unsegmented large area photovoltaic cell, the presence of a single low resistance current path could disproportionately destroy the electrical output of the entire large area cell. Obviously, the probability of damaging the entire electrical output of a large area segmented photovoltaic cell is dependent upon the surface area of that large area cell, the number of small area segments into which it is divided, and the manner in which those small area segments are electrically connected. A large area cell may therefore include a plurality of electrically interconnected, small area photovoltaic segments, each capable of producing an electrical current in resonse to illumination, said small area segments cooperating to provide the total electrical output of the large area photovoltaic cell. The term "photovoltaic segment", as used herein, will refer to one of the small area subcells, or individual subunits into which the large area photovoltaic cell is divided.
If the small area segments into which the large area photovoltaic cell is subdivided are electrically connected in series, the voltages of the individual small area segments are added to produce the voltage of the large area photovoltaic cell. In such a series-connected arrangement of small area segments, an individual shorted or shunted segment will not necessarily destroy the operation of the entire large area cell since electrical current will merely flow through the low resistance current path to the next series connected small area segment. However, the utility of dividing a large area cell into a plurality of series connected small area segments is limited by the fact that the voltage is additive. The voltage of the series connected small area segments is dependent upon the number of functioning small area segments of the large area cell. If a large number of small area segments are inoperative, the sum of the voltages of the entire large area cell may be severely reduced. Furthermore, since the output voltage of a large area photovoltaic cell having series connected small area segments is dependent upon the number of operative small area segments, damage occurring after installation of the large area cell may also cause large scale voltage loss in that large area cell. In summary, a large area photovoltaic cell which is divided into a very large number of electrically series-interconnected small area segments will not have its output voltage changed significantly by the loss of a few segments; however such a device would exhibit a high output voltage and a low output current, and thereby be of limited utility.
The use of parallel connected small area segments is an alternative approach to the division of a large area photovoltaic cell into a plurality of small area segments, which connection will prevent a small number of defective portions thereof from destroying the electrical output of the entire cell. A parallel connected array of small area photovoltaic segments will provide a relatively high output current at a voltage equivalent to that of a single small area segment, i.e. the photogenerated currents of the plurality of small area segments are additive. The use of a parallel connected array of small area segments therefore offers the advantage of a high output current. However, a major problem exists with utilizing a parallel connected array of small area segments because a single shorted or shunted small area photovoltaic segment can effectively short circuit the entire large area photovoltaic cell by providing a low resistance current path in parallel with the small area segments.
A combined series-parallel array of small area photovoltaic segments has previously been employed to form a thin film, large area photovoltaic cell, thereby overcoming the problems previously discussed with reference to series connected and to parallel connected small area segments. According to known principles, a plurality of small area photovoltaic segments may be arrayed in a plurality of rows and columns upon a substrate. Tne small area photovoltaic segments within each given row are electrically connected in series so as to form a series connected row, and those series connected rows are then electrically connected in parallel, thereby forming the large area photovoltaic cell. It should be noted that the terms "rows" and "columns" as used herein, will designate subgroups of the small area segments within the matrix array of the small area segments which combine to form the large area photovoltaic cell. The term "rows" and "columns" as used herein include any grouping of adjacent small area segments, and accordingly may be used interchangeably; for example, the series connected segments, may be described as series connected rows or series connected columns. Furthermore, it is not necessary that the rows and columns be oriented at right angles to one another; other arrangements such as diagonal arrangements may be similarly employed. In summary, the series-parallel arrangement is defined to include groups of series connected photovoltaic small area segments, generally referred to herein as rows, which rows are then electrically connected in parallel.
It should thus be apparent that the output voltage of a series-parallel arrangement of the small area segments of a large area photovoltaic cell will be determined by the voltage produced in each series connected row, while the total output current will be determined by the sum of the current produced by each of the parallel connected rows. Thus as is also readily apparent, the electrical characteristics of a large area photovoltaic cell may be selected by choosing the appropriate series-parallel arrangement of the small area segments thereof.
The importance of such a series-parallel connected large area photovoltaic cell is that it is resistant to the effects of shorts, shunts, low resistance current paths and other mechanical damage to individual small area segments. For example, if an individual small area segment develops a current shunting type of defect, it will create a low resistance current path between the two adjacent small area segments with which it is electrically connected in series. Such current shunting defect will not short circuit current from the entire large area cell as would be the case if the small area segments were connected in parallel. Although the single defective small area segment will proportionally lower the output voltage of the row of small area segments with which it is connected in series, the effect of the single defective small area segment upon the total voltage output of the large area photovoltaic cell will be negligible since there are a plurality of rows of such series connected small area segments, the rows connected in parallel. Therefore, the fractional loss in electrical performance exhibited by the large area photovoltaic cell will be equal to the number of current shunting small area segments divided by the total number of small area segments into which the large area photovoltaic cell is divided. On the other hand, damage to a small area segment of the large area photovoltaic cell which results in a complete loss of the electrical output of that small area segment (i.e. an "open circuit" rather than a short circuit), destroys the electrical output of the entire row in which that small area segment is electrically connected in series, but does not (1) impair the electrical operation of the remaining rows of small area segments of the large area cell, or (2) lower the operating voltage of the large area cell. In contradistinction, a completely series connected array of small area segments, in which an open circuit condition exists, renders the large area photovoltaic cell inoperative. It should thus be apparent from the foregoing description that the utilization of a series-parallel arrangement of small area segments in the fabrication of large area photovoltaic cells is desirable. The importance of fabricating a large area photovoltaic cell divided into a series-parallel connected array of small area photovoltaic segments should now be apparent.
Through the use of the thin metallic electrodeposited material of the instant invention, which is subsequently affixed to an insulating support member, the etching process for dividing the large area cell into discrete, electrically-insulated small area segments is more easily and more economically effected. Further, in the prior art methods of forming series or series-parallel connected small area segments, the lower electrode required a metallization (by evaporation) step, which step is obviated by employing the present process. And as still another advantage, because the insulating support member of the instant invention is affixed to the electroplated deposit following the deposition of the semiconductor material, the most economical insulating material may be used. This is because outgassing at the elevated temperatures required for semiconductor deposition no longer imposes a limitation on the type of insulating support material employed. The instant invention thereby significantly reduces problems which have heretofore limited the efficient and widespread use of thin film, large area photovoltaic devices.
A final important advantage afforded by electroformed, substantially defect-free substrate, is its ability to provide an intermediate manufacturing product which has heretofore been unavailable. Specifically, by affixing an insulating support member to the surface of the substrate opposite the substantially defect-free surface thereof (following the deposition thereonto of the body of semiconductor material and the upper electrode), an intermediate photovoltaic-ready web can be fabricated at a first production facility. The photovoltaic-ready web can then either be stored for electrical and configurational modularization to meet customer specifications, or the web can be transported to the customer or another production facility for downstream modularization. In order to provide this intermediate product capability, it is necessary that the web of photovoltaic-ready material be able to form either series connections, parallel connections or series-parallel connections. And in order to form these various types of electrical connections, it is necessary that the photovoltaic material be provided with an insulating substrate (because a common conductive substrate could only be used to fabricate parallel connections). Obviously, the development of the thin, substantially defect-free substrate described herein, the insulating material had to be of the highest quality so as to prevent the outgassing thereof during the deposition of the semiconductor material. However, with the use of the present thin substrate, it is now possible to deposit the semiconductor material and the top electrode prior to affixing the insulating support member thereto. The thin substrate can now be easily scribed along with the superjacent semiconductor material and the top electrode to provide for series connected large area cells and small segments. Further, a low quality, and therefore inexpensive, insulating material can be used.